In the petroleum refining industry, the fluidized hydrocarbon conversion and cracking of hydrocarbons is well known and may be accomplished in a variety of processes which employ fluidized solid techniques. For example, U.S. Pat. No. 2,881,130 discloses a basic fluid coking process. U.S. Pat. Nos. 3,661,543; 3,702,516; 3,759,676 and 4,055,484 disclose a fluid coking process incorporating gasification of product ore to convert heavy carbonaceous material such as petroleum residium (1050.degree. F.+) to distillate, naphtha and gaseous products in an integrated process. U.S. Pat. No. 4,325,815 discloses a catalytic fluid coking and gasification process where catalytic partially gasified coke particles are produced in situ to enhance the fundamental fluid coking and gasification process.
U.S. Pat. Nos. 3,803,023 and 3,726,791 disclose an integrated coking and gasification process in which a hydrogen rich gas is produced by steam gasification. U.S. Pat. No. 3,537,975 discloses a combination catalytic cracking and fluid coking process where heavy catalytic cracking fractionator bottoms are cracked in a transferline. The transferline effluent is discharged into the upper part of the coker reactor. The teachings of the above referenced patents are hereby incorporated by reference to illustrate a few types of fluidized hydrocarbon conversion and cracking processes, among others, to which this invention may be applied.
Normally in fluidized hydrocarbon conversion and cracking processes, relatively high molecular weight hydrocarbon liquids and/or vapors are contacted with hot, finely-divided, solid particles (e.g. in situ developed coke ore particles, catalytically enhanced coke ore particles, a mixture of coke and catalyst particles or a mixture of coke and other solid particles beneficial to the process) either in a fluidized bed reaction zone or in an elongated riser reaction zone, or some combination of these, and maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking and conversion to lower molecular weight hydrocarbons typical of those present in refinery gases, naphthas (motor gasolines) and distillate fuels boiling ranges.
During the conversion and cracking reaction, coke and feed contaminants (e.g. metals) are deposited on the solid particles in the reaction zone, thereby altering the effectiveness and activity of these solid particles for conversion and cracking reactions and the selectivity of the conversion products for producing gas, naphtha and distillate stocks. In order to restore a portion, preferably a major portion, of the effectiveness and activity to the feed coke-contaminated solid particles (spent particles), these particles are transferred from the reaction zone into other zones (heater, gasification, regeneration, etc.). Typically these zones comprise large vertical cylindrical vessels wherein the spent solid particles are maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas, such as air, under conditions to burn at least a portion, preferably a major portion, of the feed derived coke from these solid particles. The regenerated solid particles are subsequently withdrawn from these various zones and eventually, after suitable and beneficial treatment steps, reintroduced back into the reaction zone for reaction with additional hydrocarbon feed.
In processes for fluid coking, or fluid coking with gasification, commercial practice has been to employ fixed throat feed injectors. Such fixed throat feed injectors are usually designed on a forecast basis and optimized for a certain feed quality. In the actual plant operation, however, feed quality is usually different from the forecast basis, since business objectives change with time. For these reasons, most conventional fluid hydrocarbon conversion and cracking units are not exerting complete control of the reaction system.
Furthermore, it is current practice with commercial plant operations to practice multivariable constraint control to maximize refinery profits on a continuous basis. An important process variable is the reactor temperature, since it has a major impact on reactor product yield and quality. For example the naphtha and distillate products, both light and heavy gas oils, from a typical fluid hydrocarbon conversion and cracking process form a major portion of the overall refinery intermediate feed streams either to onstream blending operations or other process units such as catalytic cracking. In fact, these reactor products are probably one of the most important intermediate steps in the overall refinery upgrading processing sequence. Due to the usually large throughputs, associated with these fluid hydrocarbon conversion and cracking process units, even minor variations of reactor product yields and/or quality can have a significant impact on the operating economics of the refinery. Typically, temperature control of a commercial reactor is within a range of about .+-.10.degree. F. of set point. Improved temperature control, for example within a range of several degrees of set point, and preferably even within 1.degree. and 2.degree. F., is desirable.
Prior art methods of controlling reactor temperature in an endothermic hydrocarbon conversion and cracking process reaction system have been accomplished in various ways, keeping in mind the overall process heat balance and carbon balance must be in harmony. A typical prior art method of controlling temperature is to change the .DELTA.P (pressure drop) between the reactor and the heater vessel, which pressure drop in turn changes the circulation rate of hot coke solid particles from the heater and thus changes the reactor heat balance and corresponding temperature in the reactor. Another method of controlling temperature is to change the reactor feed temperature and heat content by means of a feed preheater, which again, will change the reactor heat balance and corresponding reactor temperature. The use of a feed preheat furnace to control reactor temperature has limits, however, since it is pegged to the hot solids (coke) circulation rate and there is a large time lag due to the large residence time of the feed in the preheater. Although controlling the solids (coke) circulation rate is more responsive, it is not very precise, since solid slumping or slugging can occur in the transport pipes. Pressure fluctuations adversely affect the flow pattern of the fluidized solid particles in the system. In addition, since the solid particles must pass through one or more valves, changing solid particle circulation rates tends to aggravate mechanical abrasion and can be disruptive to flow pattern.
According to the present invention, finer and more precise temperature control is achieved. There would be less mechanical valve wear and unscheduled shutdowns. A major advantage of the present invention is that a more tuned and more streamlined uniform fluidized solid particles flow pattern can be achieved.
There is a need for better and more continuous maximization of fluid hydrocarbon conversion and cracking processes in terms of unit profitability as well as overall refinery profitability. During the course of a two to three year plant run, there is considerable room for increasing yields and qualities of the reactor products by continuous and precise optimization of the reactor temperature control.